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Myosin heavy chain isoforms and smooth muscle function
R J Paul1, T E Hewett, A F Martin
1Department of Physiology and Biophysics, University of Cincinnati, College of Medicine, OH 45267.
Abstract:
Using isoform specific antibodies we have verified the presence of two distinct muscle type myosin heavy chain isoforms in rat uterine muscle. We have shown that an endogenous protease can cleave a small 4 kDa region from the C-terminal of the SM1 isoform which generates a pSM1 species which comigrates with the SM2 isoform on low density SDS gels. While this cleavage can complicate isoform identification, more importantly, this cleavage was associated with a substantial increase in the actomyosin ATPase. Thus we have identified a domain at the C-terminal which may be involved in regulation of the ATPase activity. Interestingly, it is at this C-terminal, tail region of the smooth muscle myosin molecule where the only known isoform specific sequence differences are located. In skinned smooth muscle fibers of rat uterine muscle, we have also shown that differences in myosin heavy chain distribution, induced by beta-estradiol treatment of ovariectomized rats, are correlated with changes in unloaded shortening velocity. Thus our work suggests that the functional significance of myosin heavy chain isoforms in smooth muscle may be similar to that observed in striated muscle.
Insights
Researchers identified two myosin heavy chain isoforms in rat uterine muscle. Proteolytic cleavage of SM1 isoform increases actomyosin ATPase activity, suggesting a regulatory role for the C-terminal domain.
Area of Science:
- Biochemistry
- Molecular Biology
- Muscle Physiology
Background:
- Smooth muscle myosin heavy chains (MHCs) are crucial for contraction.
- Understanding MHC isoform function is key to smooth muscle physiology.
Purpose of the Study:
- To investigate the presence and functional significance of myosin heavy chain isoforms in rat uterine muscle.
- To explore the impact of post-translational modifications and hormonal regulation on MHC function.
Main Methods:
- Utilized isoform-specific antibodies for myosin heavy chain identification.
- Employed SDS-PAGE to analyze protein species and cleavage products.
- Measured actomyosin ATPase activity and unloaded shortening velocity in skinned muscle fibers.
Main Results:
- Confirmed two distinct muscle-type myosin heavy chain isoforms in rat uterine muscle.
- Demonstrated that endogenous protease cleavage of the SM1 isoform generates a pSM1 species, increasing actomyosin ATPase activity.
- Observed correlations between myosin heavy chain distribution, beta-estradiol treatment, and changes in unloaded shortening velocity.
Conclusions:
- Identified a C-terminal regulatory domain in smooth muscle myosin heavy chain potentially controlling ATPase activity.
- Suggests functional significance of myosin heavy chain isoforms in smooth muscle is analogous to striated muscle.
- Highlights the impact of hormonal changes on myosin heavy chain distribution and muscle contractility.